Transistors are multi-electrode semiconductor devices in which the current flowing between two specified electrodes is controlled or modulated by the voltage applied at a third (control) electrode. Transistors fall into two major classes: field-effect transistors (FETs), and bipolar junction transistors (BJTs).
FETs include a source, a drain, and a gate. A voltage applied to the gate results in a current flow between the source and the drain of the FET through a channel that is formed beneath the gate. A commonly used FET is a complimentary metal oxide semiconductor transistor, or CMOS transistor. CMOS transistors can be either NMOS or PMOS transistors depending upon the type of semiconductive materials used to form the transistor. CMOS semiconductors include both NMOS and PMOS transistors in one semiconductor.
BJTs comprise two p-n junctions placed back-to-back in close proximity to each other, with one of the regions common to both junctions. This forms either a p-n-p or an n-p-n transistor depending upon the characteristics of the semiconductive materials used. A BJT is a three-terminal device that can controllably vary the magnitude of the current that flows between two of the terminals. The three terminals include a base terminal, a collector terminal, and an emitter terminal. The movement of electrical charge carriers that produce electrical current flow between the collector and the emitter terminals vary dependent upon variations in the voltage on the base terminal thereby causing the magnitude of the current to vary. Thus, the current flow through the emitter and collector electrodes is controlled by the voltage across the base-emitter junction.
Recently, demand for BJTs has increased significantly because these transistors are capable of operating at higher speeds and driving more current. These characteristics are important for high-speed, high frequency communication networks such as those required by cell phones and computers.
BJTs can be used to provide linear voltage and current amplification because small variations of the base-emitter voltage and hence the base current at the input terminal result in large variations of the output collector current. The transistor can also be used as a switch in digital logic and power switching applications, switching from an “off” state to an “on” state. Such BJTs find application in analog and digital circuits and integrated circuits, at all frequencies from audio to radio frequency.
BiCMOS semiconductors include BJTs and CMOS transistors manufactured in the same semiconductor.
HBTs are BJTs where the emitter-base junction is a heterojunction between semiconductor materials of different, but similarly functioning types. HBTs using compound semiconductive materials have one advantage of being manufactured using common techniques used to manufacture silicon semiconductors. Silicon/Silicon Germanium (Si/SiGe) and Silicon/Silicon Germanium Carbon (Si/SiGeC) are two types of HBTs using compound semiconductive materials which have gained popularity due to their high speed and low electrical noise capabilities coupled with the ability to manufacture them using processing capabilities used in the manufacture of silicon BJTs. HBTs have found use in higher frequency applications such as cell phones, optical fiber and other high frequency applications requiring faster switching transistors, such as satellite communication devices.
Although the compound semiconductive layer has proven useful in HBTs, once formed by existing methods, this layer is subsequently subjected to multiple thermal cycles, implantations and/or etching processes during the formation steps of the remaining elements of the HBT such as the deposition and etching of oxide layers, nitride layers and the emitter. Several of these processing steps inherently damage the compound semiconductive layer. Etching polysilicon, for example, adversely affects the compound semiconductive layer beneath the polysilicon because the ethchants used do not selectively etch only the polysilicon. Some of the compound semiconductive layer is also etched during this processing step resulting in HBTs that are relatively slower and exhibit relatively poor noise performance.
Furthermore, to improve the operating speed of a HBT, it is important that the base layer be thin enough to minimize the time it takes electronic charges to move from the emitter to the collector, thereby minimizing the response time of the transistor, and have a high concentration of dopant in order to minimize base resistance. Typically, ion implantation techniques are widely used to form a base layer. However, this technique has the problem of ion channeling, which limits the minimum thickness of the base layer. Another disadvantage of ion implantation is that the Si/SiGe or Si/SiGeC layer is often damaged by the ions during implantation. Additionally, high temperature annealing is required to drive the dopant into the material layers. This annealing step, however, alters the concentration profile within the various layers of semiconductive materials making up the transistor.
Furthermore, the differences in manufacturing techniques used to form CMOS and HBTs have made it difficult to manufacture BiCMOS semiconductors using compound semiconductive materials that have proven to be beneficial in HBTs.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.